Corrosion inhibitor for phosphoric acid



Dec. 3, 1968 w. P. BANKS CORROSION INHIBITOR FOR PHOSPHORIC ACID 2 Sheets-Sheet 1 Filed March 28. 1966 CORROSION RATE ON 1020 MILD STEEL AT |20 F 76 WET PROCESS PHOSPHORIC ACID WITH ADDED SODIUM IODIDE O O O O 2 m m mwm mmxozTIdE mham 2059200 mm zm 05G] 0.030 0040 Y WEIGHT PERCENT ADDED IODIDE (AS IODIDE ION) as U m m w w m a L m A TTOR/VEY LIQUID PHASE CORROSION RATE, MlLLl-INCHES PER YEAR Dec. 3, 1968 w. P. BANKS 3,414,376

CORROSION INHIBITOR FOR PHOSPHORIC ACID Filed March 28. 1966 2 Sheets-Sheet 2 AT I20 F IC ACID ACID PLUS SODIUM IODIDE WEIGHT PERCENT ADDITIVE 2 INVENTOR.

WILLIAM R BANKS id-2211M (Z, 4 gv 4 ATTORNE United States Patent 3 414,376 CORROSION INHIBITOR FOR PHOSPHORIC ACID William P. Banks, Ponca City, Okla, assignor t0 Continental Oil Company, Ponca City, Okla., a corporation of Delaware Filed Mar. 28, 1966, Ser. No. 537,751 6 Claims. (Cl. 23165) ABSTRACT OF THE DISCLOSURE Corrosivity of merchant grade wet process phosphoric acid toward ferrous metals is greatly reduced by addition of at least about 50 ppm. iodide ion and about 2-35 weight percent sulfuric acid.

This invention relates to inhibiting corrosion by phosphoric acid in contact with ferrous materials. In one aspect, the invention relates to a method for inhibiting corrosion by phosphoric acid comprising adding to the phosphoric acid a soluble ionizable iodide and sulfuric acid. In another aspect, the invention relates to an inhibited phosphoric acid composition comprising phosphoric acid with minor amounts of added sulfuric acid and a soluble ionizable iodide.

One of the handicaps which has resisted the widespread use of phosphoric acid is the difficulty of Shipping this material in ferrous containers. While it is possible to employ glass carboys to hold phosphoric acid, it has not been possible heretofore to ship strong phosphoric acids in iron and steel shipping containers, nor to store such acid in other than chemically resistant and consequently expensive materials.

It has been proposed to prevent the corrosion and dissolution of steel or iron drums or containers used for the storage and transportation of phosphoric acid by dissolving in the acid a material capable of precipitating a protective coating on the steel. This precipitation may be accomplished in the presence or absence of an acid regulator. The precipitated protective coating is usually a metal below iron and above mercury in the electromotive series, such as arsenic, while the acid regulator is usually an organic substance such as pyridine bases, crude anthracene, sulfite, lye, resins, wash acids from refining hydrocarbons, etc., many of which are highly colored. In previous proposals it has been necessary to decolorize the inhibited acid, in order to obtain a water-white product. By this means appreciable amounts of the inhibitor were removed from the acid.

Other inhibitors for phosphoric acid have been proposed in the art. For instance, US. Patent 1,809,041 discloses the use of various iodides, and US. Patent 3,119,662 discloses the use of sulfuric acid as a corrosion inhibitor for phosphoric acid. However, these various inhibitors have not been as effective as might be desired.

Accordingly, it is an object of this invention to provide an improved corrosion inhibitor for phosphoric acid. Another object of the invention is to provide a phosphoric acid composition of reduced corrosivity.

Other aspects, objects and the various advantages of this invention will be gained from study of this specification, the claims, and the drawing, in which:

FIGURES 1 and 2 represent the effect of various additives on the corrosion rate of phosphoric acid in contact with mild steel.

I have discovered that addition of a soluble ionizable iodide and sulfuric acid to phosphoric acid results in an acid of much lower corrosivity than when equivalent amounts of either additive are used alone. I have further found that when, according to a presently-preferred em- 3,414,376 Patented Dec. 3, 1968 bodiment of my invention, the soluble ionizable iodide is one other than potassium iodide, the common phenomenon of post-precipitation in wet-process phosphoric acid is substantially reduced.

The phosphoric acid which is treated according to my invention can be that produced by various processes, and can be of a wide range of concentration. Illustrative but not limiting of suitable types of phosphoric acid are the so-called furnace process acid and the so-called wet-process acid. Illustrative but not limiting of suitable concentrations of acid for treatment according to my process are filter acid, which usually analyzes in the neighborhood of 30-35 weight percent P 0 merchant acid, which usually analyzes in the neighborhood of 50-55 weight percent P 0 and super acid, which usually analyzes 68 weight percent P 0 or more. Because of the nature of the impurities present, wet-process acid presents the most trouble in shipping, and also comprises the large bulk of the phosphoric acid production today. My invention is especially useful in treating wet-process phosphoric acid.

The additives of this invention can be incorporated into the acid at any suitable point during the process of its manufacture. They are especially advantageously added to the newly-produced merchant acid just prior to or during its introduction into the acid storage tanks.

The sulfuric acid additive can be of any concentration, but a highly concentrated sulfuric acid is preferred simply to reduce the dilution of the phosphoric acid product. Oleum is also suitable for this use. The amount of sulfuric acid to be added can vary over a wide range, but preferably will be between about 2 and about 35, more preferably between about 5 and about 35 weight percent based on the phosphoric acid.

The soluble ionizable iodide additive can be any iodide which dissolves and ionizes, in the acid being treated, sufficiently to provide at least about 50 p.p.m. iodide ion. Presently preferred are hydrogen iodide, sodium iodide, ammonium iodide, or mixtures of these. Other examples of suitable soluble ionizable iodides include tetramethyl ammonium iodide, zinc iodide, and calcium iodide. It has been discovered that, although potassium iodide has an excellent inhibiting effect when used in conjunction with sulfuric acid, the potassium ion causes a voluminous precipitation of sludge from wet-process acid. This may be desirable in some instances where it is desirable to remove this sludge and reduce post-precipitation, but other sources of potassium ion than the iodide are cheaper, and for this reason the potassium iodide is presently less preferred. The soluble ionizable iodide should be added in amount suflicient to obtain the synergistic reduction in corrosivity in conjunction with the sulfuric acid, which is usually a mini- 'mum of at least about 50 p.p.m. based on the weight of the phosphoric acid. Addition of more than about 2 weight percent serves little purpose, since additional amounts often seem to be decomposed or in some other manner rendered ineffective.

The corrosion-inhibiting effect of these additives is sufficient that ferrous alloys in general can be used to store phosphoric acid when treated according to this invention. For instance, a 76% phosphoric acid can be treated by this invention to have a corrosion rate of 1020 mild steel at F. of 2.5 m.p.y. (milli-inches per year) Or even less.

Further understanding of this invention will be gained by study of the following example, which is intended to be illustrative and not limiting.

EXAMPLE I Coupons of 1020 mild steel were prepared and immersed in 76% wet-process phosphoric acid alone, and in admixture with varying amounts of sodium iodide, 96.5% sulfuric acid, and mixtures of the two inhibitors. Corrosion rates were determined by weight loss of the cou- 3 pons after a measured time interval. All samples were held at 120 F. during the tests. FIGURE 1 illustrates the effect of sodium iodide alone, but it should be mentioned that severe pitting was observed on these coupons. Whenever pitting occurs, the practical elfect on, e.g., a steel pipe or vessel is worse than indicated by weight loss alone. The upper curve of FIGURE 2 illustrates the effect of sulfuric acid alone. The lower curve of FIGURE 2 illustrates the effect of the combined additives. The points on the lower curve were obtained by mixing 50, 200, 400 and 400 parts per million iodide by weight, as sodium iodide, to 5, 10, 22 and 35 weight percent sulfuric acid by weight, respectively, all based on the phosphoric acid weight. One point, not shown, comprised 35 weight percent of each additive, and the resulting corrosion rate was 17 m.p.y.

EXAMPLE II Corrosion rates were determined on 1020 mild steel coupons at 120 F., in a manner similar to Example 1, in 76% wet-process phosphoric acid with no additives and with 200 p.p.m. of various iodides, using no added sulfuric acid. Corrosion rates in m.p.y. were as follows: no iodide, 1910; KI, 151; Neil, 153; NH I, 154; and HI, 153.

EXAMPLE III Corrosion rates were determined on 1020 mild steel coupons at 120 F., in a manner similar to Example I, in 76% wet-process phosphoric acid to which was added percent concentrated sulfuric acid and 200 p.p.m. of various soluble ionizable iodides. Corrosion rates in m.p.y. were as follows: no iodide, 59; sodium iodide, 10; tetramethyl ammonium iodide, zinc iodide, 21; mercuric iodide, 63; and calcium iodide, 10. Mercuric iodide, which is virtually insoluble, had no beneficial effect.

From the preceding examples, it can be seen that the combined additives result in a substantially lower rate of corrosion than when an equivalent amount of either additive is used alone.

Having thus described the invention by providing specific examples thereof, it is to be understood that no undue limitations or restrictions are to be drawn by reason thereof and that many variations and modifications are within the scope of the invention.

What is claimed is:

1. The method of reducing corrosivity of merchant grade wet process phosphoric acid toward ferrous metal which comprises adding thereto at least about p.p.m. of a soluble ioniza-ble iodide and from about 2 to about 35 weight percent of sulfuric acid based on said phosphoric acid.

2. The method of claim 1 wherein said iodide is selected from the group consisting of sodium iodide, potassium iodide, ammonium iodide and hydrogen iodide.

3. The method of claim 2 wherein said iodide is added in an amount between about 50 p.p.m. and about 2 weight percent, and said sulfuric acid is added in an amount between about 5 and about 35 weight percent based on said phosphoric acid.

4. Merchant grade wet process phosphoric acid of reduced corrosivity toward ferrous metals comprising said acid containing at least about 50 p.p.m. of a soluble ionizable iodide and from about 2 to about 35 weight percent of sulfuric acid based on said phosphoric acid.

5. The composition of claim 4 wherein said iodide is selected from the group consisting of sodium iodide, potassium iodide, ammonium iodide and hydrogen iodide.

6. The composition of claim -5 wherein said iodide is present in an amount between about 50 ppm. and about 2 weight percent, and said sulfuric acid is present in an amount between about 5 and about 35 weight percent.

References Cited UNITED STATES PATENTS 1,809,041 6/1931 Jenkins et a1. 23-165 2,567,156 9/1951 Malowan 23-165 3,044,854 7/1962 Young 23-165 3,119,662 1/1964 Viventi et al. 23-165 1,951,077 3/1934 Woodstock 23-165 OSCAR R. VERTIZ, Primary Examiner.

A. I. GREIF, Assistant Examiner. 

